The present invention relates to a heat-sealable crosslinked oriented polyethylene film and a process for producing the same, particularly to an oriented high density polyethylene film having a specific crosslinked structure, having low-temperature heat sealability which can cope with bag-making processings at high speeds, improved in antiblocking property affecting bag-making processings or filling operations of contents, and further excellent in moistureproofness and clarity, and a process for producing the same.
In order to improve the clarity of high density polyethylene films, there have been proposed various oriented polyethylene films homogeneously crosslinked by radiation or the like (Japanese Patent Publication No. 18893/1962).
Such films are improved in clarity and strength, but insufficient in moistureproofness and antiblocking property. Furthermore, the oriented polyethylene films crosslinked to such a degree that the clarity is improved have the drawback that the low-temperature heat sealability is impaired. Thus, such films are not necessarily sufficient in functions as packaging materials.
Then, the present inventors have previously proposed a crosslinked oriented polyethylene film in which the degree of crosslinking inwardly decreases across the thickness of the film (Japanese Patent Unexamined Publication No. 74820/1986).
As described above, the prior-art crosslinked oriented polyethylene films are improved in clarity and the like, but insufficient in moistureproofness. It is therefore necessary that the films are increased in their thickness or provided with barrier resin layers when used for the applications requiring the moistureproofness. For this reason, the clarity and packaging properties of the films are deteriorated, and the production cost rises. However, according to the oriented polyethylene film having the above-mentioned specific crosslinked structure, the film excellent in clarity and moistureproofness can be obtained.
However, this film alone is somewhat poor in heat sealability. For the purpose of improving the heat sealability, it has also previously been proposed to form a heat sealing layer comprising a low density polyethylene (LDPE) or an ethylene copolymer on the above-mentioned film (Japanese Patent Unexamined Publication No. 171149/1985).
On the other hand, in the packaging field, the bag-making processings at high speeds come to be increasingly required, and therefore the development of films good in low-temperature heat sealability has been desired.
According to the above-mentioned composite film, the heat sealability is improved, but there is still room for the improvement of the heat sealability in the high-speed bag-making processings (low-temperature heat sealability).
In order to improve the low-temperature heat sealability, it is necessary to increase the difference in melting point between a substrate and a heat sealing layer. In other words, it is required to select a substrate high in heat resistance and a heat sealing layer low in melting point. However, the melting point (Tm) of the crosslinked polyethylene film described above is generally from 134.degree. to 135.degree. C., so that the substrate has the limitation to the improvement of its heat resistance.
Then, for the purpose of reducing the melting point (Tm) of the heat sealing layer, the present inventors gave attention to hot-melt adhesives and studied various resins low in melting point (Tm). As such an adhesive, an adhesive essentially comprising three components of a copolymer of ethylene and vinyl acetate, liquid cis-1,4-polyisoprene rubber and a tackifier is described in Japanese Patent Publication No. 4592/1987, and the application to a polyethylene substrate is also described therein. However, the hot-melt adhesive is poor in antiblocking property, so that the bag-making processings are adversely affected. Further, when the hot-melt adhesive is used as the heat sealing layer, it is required to take care not to impair the clarity of the crosslinked oriented polyethylene film described above.